The laser gyro, developed some thirty years ago, is widely used on a commercial scale at the present time. Its principle of operation is based on the Sagnac effect, which induces a frequency difference Δν between the two optical transmission modes that propagate in opposite directions, called counterpropagating modes, of a bidirectional laser ring cavity undergoing a rotational motion. Conventionally, the frequency difference Δν is equal to:Δν=4AΩ/λLwhere: L and A are the length and the area of the cavity, respectively: λ is the laser emission wavelength excluding the Sagnac effect; and Ω is the rotation speed of the assembly.
The value of Δν measured by spectral analysis of the beat of the two emitted beams is used to determine the value of Ω very accurately.
It may also be demonstrated that the laser gyro operates correctly only above a certain rotation speed needed to reduce the influence of intermodal coupling. The rotation speed range lying below this limit is conventionally called the blind zone.
The condition for observing the beat, and therefore for the operation of the laser gyro, is the stability and relative equality of the intensities emitted in the two directions. This is not a priori an easy thing to achieve because of the intermodal competition phenomenon, which means that one of the two counterpropagating modes may have a tendency to monopolize the available gain, to the detriment of the other mode.